Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe₂O₄ Nanoparticles

Abstract

In this work, we show the enormous size effect on the ordering transition temperature, T O , in samples of CoFe 2 O 4 nanoparticles with diameters ranging from 1 to 9 nm. Samples were characterized by HRTEM and XRD analyses and show a bimodal particle size distribution centered at 3 nm and around 6 nm for "small" and "large" particles, respectively. The results and concomitant interpretation were derived from studies of the magnetization dependence of the samples on temperature at low and high magnetic fields and relaxation times using both dc and ac fields. The large particles show a typical superparamagnetic behavior with blocking temperatures, T B , around 100 K and a Curie temperature, T C , above room temperature. The small particles, however, show a colossal reduction of their magnetic ordering temperature and display paramagnetic behavior down to ∼10 K. At lower temperatures, these small particles are blocked and show both exchange and anisotropy field values above 5 T. The order of magnitude reduction in T O demonstrates a heretofore unreported magnetic behavior for ultrasmall nanoparticles of CoFe 2 O 4 , suggesting its further study as an advanced material.

Intrinsic dependence of the magnetic properties of CoFe2O4 nanoparticles prepared via chemical methods with addition of chelating agents

“…The maximum in ZFC curves observed to sucrose and oleylamine samples is, for noninteracting particles, directly proportional to the average blocking temperature which is associated with the mean particle size. On the other hand, while no maximum was observed to the glycerine sample, the oleic acid sample seems to present two maxima at T 1 ¼103 K and T 2 4350 K consistent with a bimodal distribution observed by Lopez-Dominguez et al [17] to samples grown by using a similar procedure.…”

“…Many works in literature [5,13,[21][22][23][24] which report on the magnetic properties of magnetic nanoparticles obtained via chemical methods with chelating agents discuss the changes in the magnetization saturation when compared with its bulk value as an effect coming from particles surface. We also agree with such interpretation, however, in this scenario, we want to call attention that the crude magnetization curves of the samples grown via chemical route with chelating agents can drive to the misinterpretations.…”

“…In this sense, some works in literature [17,18,[25][26][27][28][29][30] have used a generalized model to fit magnetization curves as function of magnetic field in the superparamagnetic state. It is well known that the magnetic behavior of superparamagnetic nanoparticles with uniaxial anisotropy axis is governed by magnetization reversal processes, that is, magnetic moments inside nanoparticles can rotate collectively under action of both thermal agitation and/or magnetic field.…”

“…In order to estimate the mean size and size distribution nanoparticles from the M Â H curves, following a procedure found in Refs. [17,18], we have used a uniform magnetic model based on the Langevin function weight-averaged with a log-normal particle volume distribution. In Fig.…”

“…In order to reach such goal, chemical methods are the most used due mainly to the control of the kinetics parameters and the low cost. Such optimization is generally achieved using chemical addictive (generally organic compounds) [5][6][7][8][9]. However, the low temperatures generally used in these synthesis methods are not sufficient to eliminate completely the organic contamination of desired final product.…”